Method and apparatus for cement kiln control

ABSTRACT

A control system for controlling the firing rate in a rotary cement kiln so as to maintain the temperature just ahead of the chain section at a predetermined set point. The control system includes both proportional and integral control and is adaptive in that the reset rate of the controller is increased when the temperature deviation exceeds a predetermined amount. In addition the overall gain of the control system is modified so that the gain is changed to a lower value when a temperature measurement in the mid-kiln region exceeds a certain high value with the gain being changed to a higher value when that mid-kiln temperature is below a predetermined minimum value. Means are provided to compensate for the discontinuity in the proportional control action when the mid-kiln temperature causes a change in the controller gain.

United States Patent I [1 1 Bay [ METHOD AND APPARATUS FOR CEMENT KILN CONTROL [75] Inventor: Theodosios Bay, Lansdale, Pa.

[73] Assignee: Leeds & Northrup Company, Philadelphia, Pa. 221 Filed: June 8, 1972 [21] J Appl. No.: 260,923 52 us. Cl. 432 24 [51] Int. Cl.... F27b 7/00 [58] Field of Search 432/17, 19, 24, 36, 432/51 [56] References Cited UNITED STATES PATENTS 3,578,299 5/1971 l-iurlbut 432/17 3,610,596 10/1971 Snyder 432/24 Primary Examiner-.lohn J. Camby Attorney-William G. Miller, Jr. et al.

[451 Sept. 18,1973

5? ABSTRACT A control system for controlling the firing rate in a rotary cement kiln soas to maintain the temperature just ahead of the chain section at a predetermined set point. The control system includes both proportional and integral control and is adaptive in that the reset rate of the controller is increased when the temperature deviation exceeds a predetermined amount. In addition the overall gain of the control systemis modified so that the gain'is changed to a lower value when a temperaturemeasurement in the mid-kiln region exceeds a certain high value with the gain being changed to a higher value when that mid-kiln temperature is below a predetermined minimum value. Means are provided to compensate for the discontinuity in the proportional control action when the mid-kiln temperature causes a change in the controller gain.

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l METHOD AND APPARATUS FOR CEMENT KILN CONTROL BACKGROUND OF THE INVENTION This invention relates to the production of cement in rotary kilns and is particularly directed to an improved method and apparatus for controlling thetemperature of the material being processed in a kiln so as to provide more stable kiln operation as well as a more uni- The typical rotary cement kiln is a large steel cylinder which may, at the extreme, be on the order of 25 feet in diameter and over 700 feet long. These cylinders are lined with refractory brick and are inclined in a slight angle and rotated at speeds on the order of 20 to l20 revolutions per hour. The raw material for producing the cement includes finely ground limestone, clay or shale intermixed in the proportions desired.-This mixture may be in the form of a dry pulverized mixture or in the form of a finely ground slurry. The raw material is fed into the upper or cold end of the rotary cement kiln.

As the kiln is rotated the raw materials move slowly down the kiln at a rate of speed which is a function of the kiln rotational speed. -As' the raw materials move down the kiln they pass through successive zones known as the chain zone, the preheating zone, the calcining zone, the clinkering or burning zone and the cooling zone. When the kiln is used for thewet' process, I

that is, when the feed is in the form of a wet slurry, the moisture in the slurry is evaporated in the chain zone which may extend for 25 percent of the kiln length. In the chain zone, chains are suspended from the kiln to contact the slurry and to serve as a heat exchange medium between gases and slurry to' drive off moisture.

As the raw materials move down the kiln they are heated by'a stream of hot gases produced from a burner located at the material discharge end of the kiln. Those gases and any insufflated dust, which would be introduced at the material discharge end of the kiln, flow counter to the movement of the material in the kiln as a result of the negative pressure created at the feed end of the kiln by an induced draft fan. The heat from the combustion gases as well as the heated dust which may be insufflated causes the raw material to undergo successive changes due to the steady increase in the temperature of the material as it progresses toward the discharge end of the kiln.

The temperature of the dried'raw material increases until the calcining temperature is reached. The calcining zone occupies the major portion of the kiln length.

The-temperature of the material changes little in that zone since the calcining reaction is endothermic and requires the absorption of the heat from the hot gases.

'When calcination is complete the temperature of the solid material begins to increase rapidly to a point where the exothermic clinkering reactions are initiated. The material leaves the clinkering zone at a high temperature and is thereafter cooled in the kiln and cooler.

It is well known that the degree of completion of the chemical reaction in the clinkering zone depends upon the feed composition, the temperature, and the period of time the material spends in that zone. The kiln must be controlled in such a manner so as to produce a clinker product of uniform quality. it is, therefore, desirable that the temperature gradient along the kiln length clinkering zone,as it is sometimes called. In some control arrangements such as that described in U.S. Pat. No. 3,566,091, which issued to me and my co-worker Charles W. Ross on Feb. 23, 1971, manipulation of the kiln speed is used as an alternate to manipulation of the fuel feed rate to control the burning zone temperature. Such a system may be extended where dust is being insuffiated so that the burning zone temperature may be subject to control by modifying theinsufflation of dust as a first preference and by then modifying the fuel feed rate as a second preference with control of the kiln speed as a last resort, as the range of control may require.

Applicant has found that in long kilns supplied with wet slurry it is important to maintain control of the material leaving the drying or chain zone. This will minimize the disturbances propagating down the kiln due to inconsistent drying of material in the drying zone. It has been found that the temperature just ahead of the chain section (downstream as the material flows from the chain sectiontcan be utilized as a variable which can be controlled by adjusting thefiring rate when the fir- .ing rate is not being used to control the temperature in the burning zone. When firing rate is so used, proper adaptation of the control system provides improved temperature gradients along the kiln and more constant burning zone temperatures can be maintained. It is, therefore, an object of this invention to provide animproved apparatus and method for controlling a rotary cement kiln.

- It is another object of this invention to provide an improved apparatus and method for controlling the temperature in the drying zone in a rotary cement kiln.

SUMMARY OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWING The drawing is a circuit diagram in block form showing an embodiment of the invention in analog form.

DESCRIPTION or THE PREFERRED EMBODIMENT in the FIGURE there is shown a rotary cement kiln 10 including a means 12 for introducing the raw material feed to the upper end of the kiln. This raw material feed may be in the form of a finely ground slurry which will be dried in the chain section 14 of the kiln as the material progresses toward the discharge end. The kiln 10 is shown as having a source of heat at the material discharge end by way of the introduction through the pipe 16 of fuel which is burned in the kiln, with the rate of flow of fuel and. hence the firing rate in the kiln being adjustable by varying the position of the valve 1% as by motor 20. There is also shown in the F lGURE a fan 22 which serves in conjunction with the cooler 24 to cool the clinker which is discharged from the lower end of the kiln onto cooling grate 26.

The kiln is rotated by means of motor 28 through gearing including the gears 29 and 30 whichengage the ring gear 32 around the periphery of the cylindrical kiln.

The firing rate in the kiln is controlled, as mentioned, by means of positioning motor 20 which positions valve 18 so that the position of shaft of motor 20 which couples by way of mechanical coupling 36 to the valve 18 is adjusted by the controller 38. The controller 38 is shown as providing both proportional and integral action which may be referred to respectively as proportional and reset action. The controller 38 operates to modify the flow of fuel in line 16 until the firing rate as represented, by the signal on line 40, which is derived from the flow measuring device 42 connected to measure the flow of fuel through line 16, corresponds to the firing rate set point represented by the signal on line 48.

While control systems utilized in the past have determined the firing rate PR from measurements such as measurements of the burning zone temperature as explained in US. Pat. No. 3,566,091. it has been found advantageous in very long kilns which utilize a wet slurry feed to base the control of firing rate whenever possible on the temperature measured in the area just ahead or downstream of the chain section 14 which temperature measurement will be referred to as the prechain temperature. That temperature measurement may be made, for example, by a thermocouple 50 which is subjected to the temperature of the surroundings including the raw material in the area of the kiln just before the hot gases reach the chain area 14.

The prechain temperature measured by the thermocouple 50 is then represented by a signal on line 52 which is identified as +PCT and which provides ont input to amplifier 54. The other input to amplifier 54 is a signal on line 56 identified as PCT, which represents the prechain temperature set point and which is produced from a potential source E by the adjustment of potentiometer 58.

The amplifier 54 operates to compare the signal on line 52 with the signal on line 56 so as to develop on its output line 60 a signal repersenting the control error e.

The error signal on line 60 is differentiated by the series connected capacitor 62 and the shunting resistor 64 which is connected from line 66 to ground, line 66 being an input line to the operational amplifier 68. The input to amplifier 68 from line 66 which represents dc/dt establishes a signal which represents the proportional control action for the control system operable to control the firing rate set point FR The integral or reset response for the control controlling FR is established by selectively providing another input by way of lines 70 or 72 to amplifier 68 so that amplifier 68 serves to sum signals representing the proportional and reset control actions so that a signal is provided on its output line 74 indicative of that sum.

The reset control action is selectively established with either one of two pre-established reset rates as determined by the constants K, and K, which respectively represent the settings of potentiometers 76 and 78 in input lines 72 and 70 of amplifier 68. The relay 80 is energized during that period when the prechain temperature is between its low and high limits, PCT and PCT respectively. With the relay operator 80 energized, the relay contact 80a will be positioned in contact with the fixed contact b so that the line 84 connects from the output of amplifier 54 through the potentiometer 76 to the input line 72 of amplifier 68. Thus, with the prechain temperature in its normal range of values, the reset rate is established by the constant K When the prechain temperature exceeds the high and low limits, it is desirable to increase the reset rate so as to more quickly establish by stronger control action the desired prechain temperature PCT,,. That increased reset rate is effected when the relay 80 is de-energized so that the contact 80a is in contact with the fixed contact 80c, thus connecting the line 84 through the potentiometer 78 and input line 70 to amplifier 68. The potentiometer 78, having a setting representing constant K operates to establish the reset rate represented by K The signal on line 74 which represents the sum of the proportional and reset control actions is then supplied as an input on line to integrator 92 by way of one of the potentiometers 93-95 as selected by the movable contacts actuated by the relay operators 98 and 100 which serve to determine which of the potentiometers 93-95 is in the connection between line 74 and line 90.

If, for example, the relay contact 98a is in contact with its fixed contact 98b and if at the same time the relay contact 100a is in contact with its fixed contact 100b, then the potentiometer 93, which is adjusted to represent a constant K forms the connection between line 74 and line 90 and is effective to establish the overall gain of the control system for determining the firing rate set point FR The constant K represents the normal gain for the control system and is intended to be that gain which is effective whenever the temperature measured in the mid-kiln region such as in the preheating zone, by thermocouple 102, establishes a signal on line 104 representing the mid-kiln temperature MKT which is between a predetermined low and high limit which can be respectively identified as and MKTH.

When the relay operator 98 is energized so that the relay contact 98a is operated to contact its fixed contact 980 then the overall gain of the control is established by potentiometer 94 at a value represented by K On the other hand, whenever the relay operator 100 is energized the movable contact 100a is made to contact the fixed contact 100a and the potentiometer 95 is effective to establish'the overall gain of the control system at a value represented by the setting of the potentiometer K I The use of the relay operators 98 and 100 and the associated relay contacts to modify the overall gain of the control system is a useful adaptation of the control in response to changes in the mid-kiln temperature for it has been found desirable to increase the controller gain whenever the mid-kiln temperature is below the predetermined low limit MKT and conversely, to decrease the controller gain whenever the mid-kiln temperature exceeds the high limit MKT This type of modification of the overall gain of the control system has been found to be useful since excessivelylow mid-kiln temperatures indicate an above average absorption of heat in the mid-kiln region and consequently, indicate that a greater change in the firing rate is required for each degree of deviation of the prechain temperature from its set point as compared with what would be required if the mid-kiln temperature were in its normal range of values. Similarly, when the mid-kiln temperature ex- Because of the form in which the control system is shown in the diagram, it is necessary to introduce into the integrator 92 by way of the input line 106 a signal for correcting for the discontinuous change in the proportional response of the controller in producing the output PR which results from the switching eflected by relay operators'98' and 100, which by modifying the gain of the controller in discontinuous fashion serve to modify the proportional action of the controller in a like fashion and cause a'resultant loss in proportional information, sometimes referred to as proportional windup. Thus, the signal introduced on line 106 is intended to compensate for the discontinuity in the signal resulting from the switching, resulting from relay operators 98 and 1011.

In order to introduceon line 106 the compensating signal, it is necessary to establish on line 1118 a signal representing the change in the controller gain constant with time. .Thus, amplifier 1111 is efiective to sum the inputs on lines 112*114'so as to produce on the output I line 116 of amplifier 110 a signal representing either K K or K depending upon the state of energization of relay operators 98 and 100.

If both relay operators are de-energized as shown in the FIGURE, then a potential +E, introduced at terminal 120 provides by the setting of potentiometer 122, a signal representative of K to amplifier 116. That input to amplifier 110 will effectively produce on output line 116 a signal representing the constant K When relay operator 98 is energized the movable contact 98d is operated to be in contact with its fixed contact 98:: so as to connect the potential +13 to line 1l3 by way of the sign inverting amplifier 121 and potentiorneter 126. Potentiometer 126 is adjusted to have When relay operator 106 is energized the movable contact 100d is actuated to contact the fixed contact 10th: so as toconnect the potential source +E through potentiometer 128 which is set to represent a constant AK so as to provide a signal to amplifier 1111 by way of line 116 which will produce on output line 116 a signal representing K since K plus Alt equals K The signal produced on line 1 16 is then differentiated by means of the series connected capacitor 130 and the shunt resistor 132 connected between line 108 and ground so that there is produced on line 108, a signal representing the derivative of the controller gain constaut-with respect to time. The signal on line 108 is then provided as the input to multiplier 138, the other input respect to time and that sigml serves to compensate for the proportional windup resulting from the change in the controller gain effected by relay operators 96 and 11111.

. The energization of the relay operator 66 to change the reset rate of the controller, as previously mentioned, is effected whenever the AND gate 1511 produces a positive signal on line 152. That energization of relay operator 66 will occur whenever there is simultaneously presented on the input lines 156 and 156 of the AND gate 1611, signals of negative potential from the respective comparators 156 and 166. The comparator 1611 will produce a negative signal on its output line whenever the prechain temperature represented by the signal on line 162, +PCT, has a value which is less than the value of the-signal on line 164 representing the high limit of the prechain temperature PCT as established by the setting of the potentiometer 166 in combination with the potential source E. Whenever'the signal on ,liner162 exceeds in value the signal on line 164, as

when the prechain temperature exceeds the high limit,

MKT the signal on line. 156 is positive in value and chain temperature with its low limit PCT The prechain temperature appears as a negative potential, -PCT, on line 176 which is an output of the sign changing amplifier 172 whose input from line 161 is derived from line 52 which is in turn connected to the thermocouple 56 which measures the prechain temperature. Thus, the comparator 158 compares the -PCT with the lower limit lPCT as established on the comparator input line 176, and whenever the signal on line has a value which exceeds that on line 176, indicating that the prechain temperature is above the low limit, there is a negative signal on line 154 and the relay operator 611 can be energized if simultaneously there is a negative signal on line 156. The signal PCT on line 176 is obtained by way of the potentiometer 180 which connects a potential +E to line 176. Thus,it will be evident that the logic established by the elements 160-180 as shown in the FIGURE are effective to maintain the relay $11 energized whenever the prechain temperature is between the low and high limits predetermined for that temperature, and is effective to cause the relay operator 66 to be de-energized whenever the prechain temperature is below the low limit or above the high limit.

The energization of the relay operators 98 and 100 is accomplished in accordance with the relationship of the magnitude of the signal on line 10 1, namely, the mid-kiln temperature MKT with respect to the signal established on lines 1611 and 192 representing respectively the low and high limit values MKT and MKT Thus, relay 9% is energized whenever the output 'of the comparator 266 is positive so as to produce through diode 2112 a signal causing a current flow in line 204 to the relay operator 9% which has its other connection by way of line 2116 connected to ground. A positive output from the comparator 201i is effected whenever the sig-. nal +MKT on line 210 exceeds in value the signal on line 192, thus indicating that the mid-kiln temperature exceeds the high limit previously established by the adbeing supplied from line 110, which is representative of 65 justment of the potentiometer 212 which serves to esthe error e. Thus, the signal produced on the output line of multiplier 136, namely, line 166, represents the error e times the derivative of thecontroller gain with tablish the signal on line 192 when taken in combination with the potential source E connected to terminal Similarly, the comparator 220 is effective to energize the relay operator 100 whenever the output is positive so that there is current flow through the diode 222 and line 224 to one connection for relay operator 100. The other connection to the relay operator 100by way of line 226 which is connected to ground. The output of comparator 220 will be positive whenever the signal on line 190, namely, MKT exceeds in value the signal on line 230, -MKT, indicating that the mid-kiln temperature is below the low limit MKT The signal on line 190 is obtained by means of potentiometer 232 which connects the potential source to line 190, and the signal on line 230 is obtained from line 104 by way of the sign changing amplifier 234.

Thus, it will be evident that the logic established by the elements 190-234 as shown in the FIGURE are effective to cause an energization of relay operator 98 when the mid-kiln temperature exceeds a predetermined high limit. Similarly, relay operator 100 is energized when the mid-kiln temperature is below a predetermied low limit.

Those skilled in the art will understand that a change in the gain of the proportional control action of the controller in response to the MKT exceeding its limits is an alternate approach to that shown in the FIGURE where the overall controller gain is changed. Both approaches change the effective proportional gain, that is the relationship of the proportional action taken by the controller in response to a particular deviation of the controlled variable from its set point.

While the control system illustrated in the FIGURE is shown in an analog form, it will be evident to those skilled in the art that the control of the prechain temperature in a rotary cement kiln can be carried out by utilizing the same method as set forth above through the programming of a general purpose digital computer to provide the firing rate set point FR What is claimed is:

l. A control system for controlling the temperature of the material being processed in a rotary cement kiln comprising:

means for measuring the temperature in the prechain area of the kiln, and

means for controlling the firing rate of the kiln to tend to maintain said prechain temperature at a predetermined set point.

2. A control system as set forth in claim 1 in which the controlling means includes:

means for providing proportional control action, and

means for providing reset control action.

3. A control system as set forth in claim 2 in which the reset rate of the controlling means is increased when the prechain temperature deviates from its set point by more than a predetermined amount.

4. A control system as set forth in claim 2 which includes:

means for measuring a temperature representing the temperature of the material being processed as it passes through the mid-kiln region, and

means operable to change the effective proportional gain of the controlling means to a lower value when the mid-kiln temperature exceeds a certain high value and to change said gain to a higher value when said mid-kiln temperature is below a predetennined minimum value with the effective proportional gain being at a predetermined normal value when said mid-kiln temperature is between said low and high values.

5. A method for automatically controlling the ternperature of the material being processed in a rotary cement kiln comprising:

automatically measuring the temperature in the prechain area of the kiln, and

automatically controlling the firing rate of the kilnto tend to maintain said prechain temperature at a predetermined set point.

6. A method set forth in claim 5 in which the automatic control includes proportional control action and reset control action.

7. A method as set forth in claim 6 in which the reset rate of the reset control action is increased when the prechain temperature deviates from its set point by more than a predetermined amount.

8. A method as set forth in claim 6 which includes the steps of:

automatically measuring a temperature representing the temperature of the material being processed as it passes through the mid-kiln region, and automatically changing the effective proportional gain of the controlling means to a lower value when the mid-kiln temperature exceeds a certain high value and changing said gain to a higher value when said mid-kiln temperature is below a predetermined minimum value with the effective proportional gain being maintained at a predetermined normal value when said mid-kiln temperature is between said low and high values. 

1. A control system for controlling the temperature of the material being processed in a rotary cement kiln comprising: means for measuring the temperature in the prechain area of the kiln, and means for controlling the firing rate of the kiln to tend to maintain said prechain temperature at a predetermined set point.
 2. A control system as set forth in claim 1 in which the controlling means includes: means for providing proportional control action, and means for providing reset control action.
 3. A control system as set forth in claim 2 in which the reset rate of the controlling means is increased when the prechain temperature deviates from its set point by more than a predetermined amount.
 4. A control system as set forth in claim 2 which includes: means for measuring a temperature representing the temperature of the material being processed as it passes through the mid-kiln region, and means operable to change the effective proportional gain of the controlling means to a lower value when the mid-kiln temperature exceeds a certain high value and to change said gain to a higher value when said mid-kiln temperature is below a predetermined minimum value with the effective proportional gain being at a predetermined normal value when said mid-kiln temperature is between said low and high values.
 5. A method for automatically controlling the temperature of the material being processed in a rotary cement kiln comprising: automatically measuring the temperature in the prechain area of the kiln, and automatically controlling the firing rate of the kiln to tend to maintain said prechain temperature at a predetermined set point.
 6. A method set forth in claim 5 in which the automatic control includes proportional control action and reset control action.
 7. A method as set forth in claim 6 in which the reset rate of the reset control action is increased when the prechain temperature deviates from its set point by more than a predetermined amount.
 8. A method as set forth in claim 6 which includes the steps of: automatically measuring a temperature representing the temperature of the material being processed as it passes through the mid-kiln region, and automatically changing the effective proportional gain of the controlling means to a lower value when the mid-kiln temperature exceeds a certain high value and changing said gain to a higher value when said mid-kiln temperature is below a predetermined minimum value with the effective proportional gain being maintained at a predetermined normal value when said mid-kiln temperature is between said low and high values. 